Are time and temperature the only constraints to the simulation of organic matter maturation?

Abstract

Confined pyrolysis, hydrous pyrolysis and high pressure hydrous pyrolysis were performed with various samples (Toarcian shale and kerogen, Woodford shale and kerogen, Manakam coal) and under different experimental conditions, in order to test the effects of pyrolysis variables on the simulation of maturation of organic matter. Results show that: (i) experimental time and temperature are not exchangeable parameters as suggested by first order kinetic law models; (ii) increasing confining pressure (300–1300 bars) in gold cell experiments performed on Woodford samples has a limited influence on the reduction rate of the oil potential, the TSOM (Total Soluble Organic Matter) yield and the structure of the polars; (iii) the presence of excess liquid water (0–100 weight %) in confined pyrolysis experiments does not significantly influence organic matter maturation. However, comparisons with classic hydrous pyrolysis (200 weight % water) could indicate major differences in timing and composition of the TSOM (bitumen + expelled fraction) as well as of the solid residue. It appears that excess water is not necessary to simulate organic matter maturation when the system is sufficiently confined. Hydrous pyrolysis conditions (low partial pressure of products) delay Woodford kerogen conversion. In addition, results concerning the Mahakam coal suggest that the structure of the organic matter plays an important role concerning the water effect; (iv) increasing water pressure in hydrous conditions (220–1300 bars) drastically lowers and delays Woodford kerogen maturation. Quantitative analysis of the water produced during confined pyrolysis and comparison of the extent of aromatization in hydrous and confined pyrolysis suggests that two competing hydrogen transfer mechanisms occur during organic matter pyrolysis. This study shows that additional parameters must be defined when comparing the results of the experimental simulation of organic matter maturation.